TRANSMISSION ELECTRON MICROSCOPY (TEM) INVESTIGATION OF MINERALOGICAL COMPOSITION OF OLDUVAI’S LOCALITY 80 CORE SAMPLES

A set of samples from Olduvai’s Locality 80 were investigated by TEM techniques in order to determine mineralogical diversity of the clay mineral assemblage to help refine paleoclimate aridity reconstructions. Samples show a great deal of mineralogical diversity and little evidence for diagenesis. A variety of Mg-smectite, montmorillonite, probable illite, jarosite, iron-oxides, gypsum, titanium oxides and kozulite have been observed. Thus far samples investigated are dominated by Mg-rich smectites. Multiple morphologies of Mg-rich smectite occur. TEM data supports the interpretation that there is not extensive secondary mineral alteration or diagenesis. This is a critical finding for interpreting mineralogical and chemical results in the context of paleoclimate. However there have been some limited observations of sepiolite – smectite transformation(s) or intergrowths similar to textures observed by Krekeler et al. (2005) and Golden and Dixon (1990). Aggregates displaying this texture compose 5 to 10% of the sample grid investigated and these textures so far have only been observed in the 11 m depth material. The textures in Olduvai’s Locality 80 samples are interpreted as representing a reaction from the modulated phyllosilicate to the smectite phase. Several other non-phyllosilicate minerals are observed that are likely volcanic ash or aeolian detritus. These include feldspars, quartz, and Fe-oxides. Other phases include hornblende and Ti-V and Ti-V-Fe oxides. Variations of V concentration in bulk chemical analysis may be a good proxy for detrital input. Opal and dolomite were observed, providing constraints on the geochemical conditions of the water column. The presence of opal particles suggests that Mg was depleted locally or temporarily and conditions fluctuated near the stability field boundary of phyllosilicates and opal. Dolomite occurrence indicates there may have been Mg in excess or conditions fluctuated near the stability field boundary. TEM results provide constraints for the interpretations of paleoclimate reconstructions by showing phyllosilicates appear to be not extensively modified by diagenetic processes and detrital phases can be identified. This ultimately aids interpretations of aridity based on clay mineral assemblages.